The present invention relates to a bidirectional optical semiconductor apparatus, and more particularly relates to a bidirectional optical semiconductor apparatus with enhanced optical isolation performance.
In recent years, a fiber-to-the-user system has been proposed for transmitting data from a base station to home users by way of optical fibers. In the fiber-to-the-user system, optical communication is established bidirectionally by providing optical transmitters and receivers for both the base station and the home users. Accordingly, an optical transmitter/receiver apparatus, including: an optical transmitter; an optical receiver; and an optical transmission line, is required. The optical transmitter may be a semiconductor light-emitting device for outputting an optical signal. The optical receiver may be a semiconductor light-receiving device like a photodiode for receiving the optical signal. And the optical transmission line may be a bundle of optical fibers for connecting the optical transmitter and receiver together.
In a prior art optical transmitter/receiver apparatus, an optical fiber for an optical transmitter, including a semiconductor laser device assembled in a package, is coupled to another optical fiber for an optical receiver, including a light-receiving device assembled in another package, via a coupler. Hereinafter, this apparatus will be called a "first conventional optical transmitter/receiver apparatus" for convenience of explanation.
Although commercially available optical transmitter and receiver can be used for this apparatus, such an apparatus is disadvantageous in view of downsizing and cost reduction, because separate optical transmitter and receiver should be coupled together via a coupler.
In order to solve such a problem, another prior art optical transmitter/receiver apparatus has a planar lightwave circuit (PLC) structure. The apparatus includes: a quartz substrate; semiconductor laser and light-receiving devices integrally supported by the substrate; and an optical waveguide formed within the substrate. Hereinafter, this apparatus will be referred to as a "second conventional optical transmitter/receiver apparatus".
In the second conventional optical transmitter/receiver apparatus, downsizing is realized to a certain degree by integrating the semiconductor laser and light-receiving devices on a single quartz substrate. However, since the area of the optical waveguide formed within the substrate can be no smaller than a certain limit, the size of the apparatus still cannot be regarded as sufficiently small. In addition, since the PLC should be connected to optical fibers, the cost effectiveness thereof is not totally satisfactory, either.
Thus, the present inventors proposed a bidirectional optical semiconductor apparatus, such as that shown in FIG. 9, in PCT International Application No. WO97/06458. Specifically, a silicon substrate 2 is placed on the bottom of a package 1 on the left-hand side in FIG. 9. And a semiconductor laser device (i.e., an exemplary semiconductor light-emitting device) 3, for emitting "signal light" at a wavelength of 1.3 .mu.m, for example, is fixed onto the upper surface of the silicon substrate 2. In this specification, the "signal light" means an optical signal, which is output from an optical transmitter, propagated through an optical waveguide and then received by an optical receiver in the form of light all through these processes. In the following description, the "signal light" in this sense will be simply referred to as "light", unless explicitly stated otherwise. On the right-hand side in FIG. 9, a glass substrate (i.e., an exemplary substrate) 4, having a groove extending in the direction of the optical axis, is placed on the bottom of the package 1. An optical fiber 5 (i.e., an exemplary optical waveguide) is embedded within the groove of the glass substrate 4 and one end of the optical fiber 5, closer to the laser device, is fixed onto the upper surface of the silicon substrate 2. A half mirror (i.e., an exemplary optical branching filter) 6 is inserted into the glass substrate 4 such that a predetermined angle is formed between the mirror 6 and the optical axis. The half mirror 6 transmits about 50% of the output light, incoming from the left of the optical fiber 5 at the wavelength of 1.3 .mu.m, and reflects upward about 50% of the input light, incoming from the right of the optical fiber 5 also at the wavelength of 1.3 .mu.m, on the upper surface of the glass substrate 4, a light-receiving device 7 such as a photodiode (i.e., an exemplary semiconductor light-receiving device) of a surface-receiving type is fixed for receiving the input light, reflected by the half mirror 6, and outputting photocurrent.
Since the substrate embedding the optical waveguide, the semiconductor light-emitting device and the optical branching filter are integrated within a single apparatus, the bidirectional optical semiconductor apparatus is advantageous in improving the ease of use and reducing the overall size thereof. However, this bidirectional optical semiconductor apparatus still has a problem regarding optical isolation. Specifically, since the light, emitted from the semiconductor light-emitting device, is partially received by the semiconductor light-receiving device, optical isolation adversely deteriorates.
During a relaxation time of certain duration immediately after the semiconductor light-emitting device has been driven with a pulse, the output light, emitted from the semiconductor light-emitting device, leaves some tailing light behind. Thus, with the light-emitting and light-receiving ends insufficiently isolated optically, when the input light is received by the semiconductor light-receiving device, the tailing component of the output light, emitted from the semiconductor light-emitting device, is also received by the receiving device to form noise with respect to the incoming light. As a result, the performance of the semiconductor light-receiving device deteriorates. Accordingly, in performing bidirectional optical communication, improvement of optical isolation plays a crucial role.
In the first conventional optical transmitter/receiver apparatus, deterioration in optical isolation is allegedly suppressed by coupling the optical fiber on the transmitting end to the optical fiber on the receiving end with an offset provided at the coupler.
In contrast, no measures have been taken to suppress the deterioration in optical isolation in the second conventional optical transmitter/receiver apparatus and in the bidirectional optical semiconductor apparatus disclosed by the present inventors in the above-identified patent application.